1. Technical Field
The disclosed embodiments relate to key scanning, and more particularly relate to key scanning of key matrices having large numbers of keys.
2. Background Information
Cellular telephones have keys that a user can press to use and control the telephone. A circuit is needed to determine which one of the keys the user has pressed. FIG. 1 (Prior Art) is a simplified diagram of conventional cellular telephone key scanning circuitry. The circuitry includes a baseband processor integrated circuit 1 and a key matrix 2. Key matrix 2 includes multiple horizontally extending row conductors 3-6 and multiple vertically extending column conductors 7-10. A key is associated with each intersection of a row conductor and a column conductor. If a key is pressed by the user, then the corresponding row conductor is coupled to the corresponding column conductor. In operation, key scanning circuitry of baseband processor 1 scans through the row conductors, driving a signal onto each of the row conductors 3-6 one by one and detecting if the signal is present on any of the column conductors 7-10. If, for example, key 11 is pressed, then when baseband processor 1 drives row conductor 4 it will detect a signal on column conductor 8. The row conductor onto which the signal was driven and the column conductor upon which the signal was detected indicate which one of the keys was pressed. For cost reasons, it is desirable to reduce the number of terminals on the baseband integrated circuit 1. Reducing the number of connections between the baseband processor and the key matrix is therefore desirable.
FIG. 2 (Prior Art) is a simplified diagram of another conventional cellular telephone circuit. A dedicated key scan controller integrated circuit 12 performs the key scanning function and senses which one of the keys of key matrix 13 has been pressed. Key scan controller 12 then forwards information on which key was pressed via a serial connection 14 to baseband processor integrated circuit 15. Due to the serial interface between baseband processor integrated circuit 15 and key scan controller 12, the number of terminals on baseband processor integrated circuit 15 is reduced in comparison to the circuit of FIG. 1. A disadvantage, however, is that the key scan controller integrated circuit 12 is another integrated circuit that must be provided in the cellular telephone. Providing the additional integrated circuit, which is typically a microcontroller or a dedicated key scanning state machine, adds an undesirable and nontrivial cost to the cellular telephone.
FIG. 3 (Prior Art) is a simplified diagram of another conventional cellular telephone circuit. A separate discrete resistor is disposed between a first common node 16 and each of the row conductors 17-20 of key matrix 21. These separate discrete resistors are designated in FIG. 3 with reference numerals 22-25. Similarly, a separate discrete resistor is disposed between a second common node 26 and each of the column conductors 27-30 of the key matrix. These resistors are designated in FIG. 3 with reference numerals 31-34. A low-power resistance-change detecting circuit in baseband processor 35 monitors the matrix for a change in resistance between the first and second common nodes. If a change in resistance is detected, then the low-power resistance-change detecting circuit enables a more accurate sensing circuit. The more accurate sensing circuit measures the resistance between the SENSE+ and SENSE− terminals of the baseband processor integrated circuit 35. Each key when pressed results in a unique resistance between the SENSE+ and SENSE− terminals. Accordingly, the resistance measured by the accurate sensing circuit is used to identify the key that was pressed. For example, if key 36 is pressed, then row conductor 18 is coupled to column conductor 28 such that a total series resistance of 5R is present between the SENSE+ and SENSE− terminals of baseband processor integrated circuit 35.
FIG. 4 (Prior Art) is a table that shows the resistance between the SENSE+ and SENSE− terminals of the baseband integrated circuit 35 for each possible key that can be pressed. Note that the 5R resistance is unique and identifies the key pressed to be the “5” key 36.
A disadvantage of the circuit of FIG. 3 is that the total number of keys for which key presses can be sensed reliably is limited. Depending on the type of key matrix and on the types of resistors used and on the accuracy of the current measuring circuitry within baseband processor 35 and other factors, the maximum number of keys that can be reliably detected can be less than sixty, or can be less than fifty, and in some cases is around thirty. Some cellular telephones include more than thirty keys. For example, some cellular telephones include standard QWERTY keyboards involving approximately fifty keys. The circuit of FIG. 3 may not work well or at all with a key matrix having so many keys.